The several methods now widely used commercially for cutting gears from gear blanks, i.e., hobbing, shaping, and milling with rotary form cutters, are all based upon the use of a dedicated tool which will cut only teeth of a single form and size.
Hobbing is a continuous process in which the involute profiles of the teeth of cylindrical gears are generated by the rotation of a series of helically arrayed cutters whose individual cutting edges sweep a conical path. In this arrangement, the cutting speed of the tool and the generating movement of the tool with respect to the gear blank are interdependent, and the gear teeth are generated incrementally about the entire periphery of the gear blank as the cutter is slowly fed axially of the rotating gear blank.
In disc shaping, or gear shaping, the reciprocating cutter itself is in the form of an involute gear, and both the shaping cutter and the gear blank are incrementally indexed by rotating both with the same pitch-surface advance before each cutting stroke after the cutter has entered the gear blank radially to the desired cutting depth.
With the rack shaping method, i.e., where the reciprocating tool assumes the form of a rack to be meshed with the gear to be formed, the involute tooth profile is also generated by incremental indexing rotation of the gear blank with concurrent tangential index of the rack equal to the pitch circle index of the gear blank before each stroke of the cutter after the rack has entered the gear blank radially to the necessary depth. The process differs from disc or gear shaping in that the length of the rack cutter is limited by practical considerations, and requires tooth-indexing of the gear blank relative to the cutter.
Both disc and rack shaping are intermittent processes as the tool in each case cuts only on the forward stroke and is idle on the return.
Milling with rotary form cutters, i.e., an axial or helical cut with cutting edges shaped to the involute profile to be left upon the gear teeth, likewise requires tooth indexing of the gear blank relative to the cutter. In some instances, slot milling with an ordinary cutter is employed as a preliminary roughing operation to be followed by a finishing operation with a rotary cutter having the correct involute form, or by hobbing or rack shaping.
Compared with each other, these prevailing methods have their advantages and disadvantages. For example, the cutting paths swept by hobs, being conical surfaces of revolution, leave helical scallops on the profile of the tooth. The resulting surface of the profile may be undesirably erose if the axial feed of the hob during or between successive passes is not limited, resulting in relatively slow production due either to the limited axial feed required for the sake of acceptable finish, or to the subsequent shaving or grinding operation which may be required to achieve it.
The primary common disadvantage, however, of all of these cutting methods is their reliance upon the concept of basic racks having standardized tooth and tooth-space proportions and pressure angles. The hob, the disc shaper, the rack shaper, and the rotary form cutter embody a single tooth form dictated by one of the basic racks. A different tool is therefore needed for each variation in diametral pitch, circular pitch, and metric module; for each variation in pressure angle; for each variation in depth proportions, whether of full depth or one of the stub tooth variations; for each variation of root-fillet radius; and, finally, for each variation of function in the production sequence, i.e., roughing, pre-grind, pre-shave, or finishing.
In addition, different tools are required in some systems to adapt them for helical gears, and even for the hand of the helix, right or left.
Bevel gears, whether straight-toothed or helical, require still different machines and tooling systems.
Moreover, as the design of gear teeth is to some degree the compromise of conflicting criteria, the relatively complex calculations involved in resolving them, combined with the cost of tool inventory for gear-cutting systems premised on the rack form, has led to the development of standard data for standardized gears which has put gear design into fairly rigid confinement.